Triple-negative breast cancer (TNBC), characterized by breast tumors lacking estrogen receptor (ER), progesterone receptor (PR) and HER2, comprises 15-20% of all breast cancers. TNBC mostly comprises the basal-like molecular subtype of breast cancer and has distinct clinical and pathologic features. TNBC represents an important clinical challenge because of its aggressive behavior and poor overall survival; and patients with TNBC have a high risk of relapse and death. Currently, targeted therapies for TNBC are lacking, leaving chemotherapy as the mainstay of treatment. Hence identification of new risk biomarkers for TNBC is needed to prevent the disease relapse and to improve treatment and care for patients with TNBC. In this proposal, we seek to determine whether the serine/threonine protein kinase D3 (PKD3) is a novel biomarker for TNBC. PKD family kinases include PKD1, PKD2 and PKD3. We have recently shown that PKD2 and PKD3 are two major PKD isoforms expressed in breast cancer cells. Unlike PKD2, PKD3 expression varied among breast cancer cell lines. We found that PKD3 was highly expressed in tumorigenic TNBC cell lines (HCC1806 and MDA-MB468) and in T47D breast cancer cells stably expressing epidermal growth factor receptor (EGFR). In contrast, PKD3 protein level was low in ER-positive and HER2-overexpressing breast cancer cell lines. By using PKD isoform-specific siRNAs, we have identified PKD3 as an important isoform mediating PKD signal transduction and cell proliferation in tumorigenic HCC1806 TNBC cells. We further showed that breast cancer cells expressing a high level of PKD3 were more responsible to PKD inhibitors compared with HER2- overexpressing breast cancer cells that have a low PKD3 level. On the basis of these findings, we hypothesize that PKD3 may be an important growth regulator and risk biomarker for breast cancer, especially for aggressive TNBC. We will test this hypothesis with following two Specific Aims: Aim 1) To determine the expression of PKD3 in breast tumors and correlate its expression with breast cancer subtypes and tumor malignancy; Aim 2) To determine the role of PKD3 in the growth and signaling of TNBC in vivo. We anticipate that this proposal will provide valuable direct evidence to assess whether the aggressive TNBC expresses more PKD3 compared with other breast cancer subtypes and whether PKD3 is a critical growth regulator of TNBC in vivo. These studies may lead to a finding that PKD3 could be a novel risk biomarker for TNBC.